I run a Win machine, but should not make any difference.
If Bridge needs 2 separate caches it will make them. In previous versions they were not compatible. Not sure now as Bridge has gone from 32 bit to 64 bit programing, so what this means for the cache I don't know. I saw one post in the Photoshop forum where user feels CS6 overwrites CS5 cache, but that is more of a situation in RAW where ACR 7.x uses a new profile.
I am reading you have the cache manually set to 2 separate locations?
Yes at present I have the original cache that was created by CS5.1 and then a second one that was created by CS6. These are stored by default in the User's Library but I read somewhere that performance was enahnced if they were moved to the same folder as the images, so I did that did that manually.
Apart from the space saving it occured to me that a shared cache would mean both versions stayed up to date but I wonder about the difference between the two different Raw processing engines and suspect that would be the biggest block against using a shared cache. I can't remember ever seeing this point addressed by Adobe so it would be good to have an authoritative answer one way or another.
As a test I just selected the folder for Bridge CS6 as the cache folder for CS5.1 as well. Upon restarting Bridge 5.1 it started to rebuild the previews of the images inside Bridge rather than popping up immediately, so I am not quite sure what that means in practice or why it would need to do that if it wasn't writing over the existing CS6 cache. When I reopened Bridge in CS6 it all looked normal and opened immediately with no further activity. In this case the difference in processing engine isn't creating a problem because any Raw file I click on open automatically in ACR CS6.